Collective modes in two- and three-dimensional electron systems with Rashba spin-orbit coupling

TL;DR

This paper investigates the collective charge and spin excitations in 2D and 3D electron systems with Rashba spin-orbit coupling, revealing mode couplings, damping effects, and optical signatures relevant for experimental detection.

Contribution

It provides a comprehensive analysis of charge and spin mode dispersions, their coupling, damping mechanisms, and optical responses in Rashba systems across dimensions, including experimental implications.

Findings

Charge plasmons couple to one chiral-spin mode at finite wavenumber.

Landau damping suppresses chiral-spin modes in 3D due to gapless particle-hole continuum.

SOC induces finite-frequency spectral weight in optical conductivity, satisfying sum rules.

Abstract

In addition to charge plasmons, a 2D electron system with Rashba-type spin-orbit coupling (SOC) also supports three collective modes in the spin sector: the chiral-spin modes. We study the dispersions of the charge and spin modes and their coupling to each other within a generalized Random Phase Approximation for arbitrarily strong SOC, and both in 2D and 3D systems. In both 2D and 3D, we find that the charge plasmons are coupled to only one of the three chiral-spin modes. This coupling is shown to affect the dispersions of the modes at finite but not at zero wavenumbers. In 3D, the chiral-spin modes are strongly damped by particle-hole excitations and disappear for weak electron-electron interaction. Landau damping of the chiral-spin modes in 3D is directly related to the fact that, in contrast to 2D, there is no gap for particle-hole excitations between spin-split subbands. The gapless continuum is also responsible for Landau damping of the charge plasmon in 3D - a qualitatively new feature of the SOC system. We also discuss the optical conductivity of clean 2D and 3D systems and show that SOC introduces spectral weight at finite frequency in a such way that the sum rule is satisfied. The in-plane tranverse chiral-spin mode shows up as dispersing peak in the optical conductivity at finite number which can can be measured in the presence of diffraction grating. We also discuss possible experimental manifestations of chiral-spin modes in semiconductor quantum wells such InGaAs/AlGaAs and 3D giant Rashba materials of the BiTeI family.